1. Field of the Invention
The present invention relates to promoted sulfur curing systems for elastomer compositions containing halogenated elastomeric copolymers of a C.sub.4 to C.sub.7 isomonoolefin and para-alkylstyrene.
2. Description of the Related Art
Halogenated copolymers of isobutylene and up to about 4 mole % of isoprene (butyl rubber) are well known polymer materials whose vulcanizates offer some outstanding properties not possessed by many other diolefin based elastomers. Articles prepared from many cured halogenated elastomers offer improved resistance to oils and greases as well as resistance to oxygen and ozone degradation. Butyl rubber vulcanizates exhibit good abrasion resistance, excellent impermeability to air, water vapor and many organic solvents, as well as resistance to aging and sunlight. These properties render these materials ideal candidates for one or more applications such as water hoses, organic fluid hoses, components in tire construction, gaskets, adhesive compositions and various molded articles.
More recently, a new class of halogenated elastomeric interpolymers have been discovered which offer many of the same properties as halogenated butyl rubber, but are even more ozone and solvent resistant. These materials are the halogenation product of random copolymers of a C.sub.4 to C.sub.7 isoolefin, such as isobutylene, and a para-alkylstyrene comonomer wherein at least some of the alkyl substituent groups present in the styrene monomer units contain halogen.
More preferred materials are elastomeric copolymers of isobutylene and para-methylstyrene containing from about 0.5 to about 20 wt. % para-methylstyrene wherein up to about 65% of the methyl substituent groups present on the benzene ring contain a bromine or chlorine atom, preferably a bromine atom. These copolymers (hereafter referred to as HI-PAS) have a substantially homogeneous compositional distribution such that at least 95% by weight of the polymer has a para-alkylstyrene content within 10% of the average para-alkylstyrene content of the polymer. They are also characterized by a very narrow molecular weight distribution (Mw/Mn) of less than about 5, more preferably less than about 2.5, viscosity average molecular weights in the range of from about 300,000 up to about 2,000,000, and a glass transition temperature (Tg) of below about 50.degree. C.
These copolymers may be prepared by slurry polymerization of the monomer mixture using a Lewis Acid catalyst, followed by halogenation, preferably bromination, in solution in the presence of halogen and a radical initiator such as heat and/or light and/or a chemical initiator.
Preferred brominated copolymers generally contain from about 0.1 to about 2 mole % of bromomethyl groups, most of which is monobromomethyl, with less than 0.05 mole % dibromomethyl substituents present in the copolymer. These copolymers and their method of preparation are more particularly disclosed in U.S. Pat. No. 5,162,445, the complete disclosure of which is incorporated herein by reference.
The aromatic halomethyl groups present in such copolymers permit facile cross linking to be accomplished in a variety of ways, including by means of zinc oxide or promoted zinc oxide curing systems normally used to cure halogenated butyl rubber. However, because these polymers do not contain ethylenic or allylic unsaturation, they are not normally curable using conventional sulfur or sulfur-containing curatives such as are used to cure diolefin elastomers, e.g., natural rubber, butadiene or isoprene based rubbers.
One important application for HI-PAS elastomers is their use as a blend component with sulfur curable unsaturated elastomers, e.g., natural rubber or polybutadiene, which normally lack good aging properties, ozone resistance, solvent resistance and the other physical properties inherent in HI-PAS elastomers. Optimization of the properties of such blends relies on suitable morphology of the blend and particularly on good co-vulcanization between the polymer components. However the cure of HI-PAS proceeds by a different chemical mechanism than the cure of unsaturated general purpose rubber (GPR) cured with sulfur, and it is generally necessary to include independent curing systems into such blends, one which is a cure for the HI-PAS elastomer and a sulfur system which is a cure for the unsaturated GPR. The lack of significant intervulcanization between the different polymer phases results in a diminution of important properties in cured articles prepared from such blends such as tensile, modulus, tear strength and resistance to chemical attack, as well as laminar adhesion between rubber stocks containing these different polymer phases.
It has been proposed in the prior art that brominated isobutylene/para-methylstyrene copolymers may be reacted with a tung oil acid (9, 11, 13-octadecatrienoic acid) in a nucleophilic substitution reaction such that a majority of the benzylic bromine is replaced to yield polymer-bound acid ester groups, as disclosed in Example 112 E of U.S. Pat. No. 5,162,445. It is disclosed that the resulting attached conjugated unsaturation permits facile vulcanization of the polymer and covulcanization of the polymer with unsaturated rubber using sulfur vulcanization systems. A disadvantage of this process is that it requires isolation of the tung oil acid and a complicated prereaction thereof with the brominated copolymer, which can add considerable complexity and expense to the preparation of the rubber.
Tung oil per se is also known as an additive to various curable rubber formulations to improve the properties of vulcanizates. For example, Japanese publications J01221441 and J01215838 disclose that the addition of 5-20 parts of tung oil to vulcanizable SBR formulations improves tire tread friction properties. J01118551 and J01118548 disclose that tires having improved anti-cut index are achieved by including about 3-5 parts by weight of tung oil into the vulcanizable composition. U.S. Pat. No. 3,169,566 discloses that a variety of unsaturated vegetable oils, including tung oil, reduce the surface tackiness, and hence improve the dirt resistance of light colored halogenated butyl rubber stocks such as white tire sidewalls.